1. Field of the Invention
The conventional method for producing powder coatings as shown in FIG. 12 consists of weighing and mixing the powder coating ingredients, then extruding the mixture to obtain a homogeneous melt mixed product. The melt mix is compressed into a sheet form, cooled, flaked, then ground into a powder form which is passed through a screener to remove oversize particles prior to packaging.
2. Description of the Related Art
The average particle size of coating powders for electrostatic spray applications as supplied by the powder coating manufactures, generally range from twenty-five to fifty microns. Any given powder coating product will be ground to a specific average particle size (e.g. 35 microns) and ultimately be shipped to various coaters for application. Each coater, however, may require a slight to vastly different particle size than that supplied in order to achieve a desired thickness and/or appearance. Each commercial powder coating application system may also perform better with a particle size different than that supplied by the powder coating manufacturer.
The disadvantage of this method is that some powder paints are apt to sinter during transporting to the customers site or during storage. The sintering phenomenon occurs when the ambient temperature is high and too close to the glass transition (Tg) point of the base resin system utilized in the coating or the particle size of the ground powder is too fine (&lt;20.mu.). This could necessitate the need to store the powder in refrigerated rooms.
In fact, according to an experiment by the present inventors, sintering phenomon was observed when a low Tg (=45.degree. C.) powder coating with mean particle size 30 .mu.m having a composition of glycidyle group contained acrylic resin 100 gr., decane di-carboxylic acid 25 gr. and an additive agent 1 gr. and super-fine powder coating with mean particle size 10 .mu.m having the substantially identical composition except for Tg=55.degree. C. of the resin were kept at 35.degree. C. for two months.
The grinding-at-the-gun technique allows the chip (flake) that is formed after the extrusion process to be packaged and sent directly to the coating site. The term, pellet, chip or flake, refers to the extruded powder coating mixture that has been compressed into a thin sheet, cooled to below the melt-mix freeze point and broken into small chip form by means of a mechanical crusher.